On the area of 3,100 ha of cultivated fields the density of the following species of rodents was estimated: M. arvalis in fields and C. glareolus, A. flavicollis and A. agrarius in woods. At the same time the numbers of predatory nammals and birds were estimated. During the whole period of study in area under examination 3,400,000 individuals of M. arvalis and 43,000 of forest rodents were present. Between the depression and the peak of numbers the density of M. arvalis increased over one hundred-times. At the density of 44 individuals/ha the voles consumed in a year 79 kg d.w. of alfa-alfa/ha, and at 774 indiv./ha as much as 1,153 kg d.w./ha. It was demonstrated that the ratio of plant weight destroyed to consumed amounted in alfa-alfa cultures to 1.8:1. At a low density c\f M. arvalis considerable losses of winter rye during winter are compensated by the regrowth of plants in spring. Food requirement of predators was determined and their diet in field conditions was investigated. The highest number of M. arvalis was consumed by foxes, lower by cats and weasels. The predators hunted mainly in areas showing the highest rodent density. With the increased numbers of M. arvalis its share in the diet of all predators rose up to 80%. Despite this the pressure of predators on the population of M. arvalis decreased, e.g. in the year of outbreak the predators removed barely 10.8% of all present individuals. With the increased proportion of M. arvalis in the diet of fox the proportion of consumed hares decreases. A scheme of trophic relationships of M. arvalis is presented. INTRODUCTION AND STUDY AREAComprehensive understanding of the importance of the common vole, Microtus arvalis (Pallas, 1779), in the trophic relationship of cultivated fields is still incomplete despite a large number of ecological studies concerning this species, as manifested by the publication of two monographs (Kratochvil, 1959; B a s e n i n a, 1962). Existing historical notes indicate that since ancient times M. arvalis has been recognized * Praca została wykonana w ramach problemu węzłowego 09.1.7., koordynowanego przez Instytut Ekologii PAN. Acta theriol. 9 r. 2 si

The aim of investigations was to elaborate an accurate and laboursaving method of estimating the numbers of small rodents. Each of 8 two-week series of captures consisted of: (1) a seven-day period of prebaiting the rodents on an area of 5.76 ha, over which a grid of 256 points with bait was arranged and (2) a seven-day period of intensive removal of rodents by means of snap-traps, two of which were set on each point. No correlation was found between the number of baits consumed and the number of rodents on a given point. From 78-100 per cent of all rodents captured over a seven-day period were removed by the fifth day. Differences were found in the rate of removal of different species of rodents and of marked and unmarked individuals of the same species. Estimate of population numbers was made by the regression method, obtaining satisfactory results in 17 cases out of 18 possibles. This method is recommended, but it is suggested that the prebaiting and removal periods should be shortened to five days each.

In long term studies the following climatological characteristics were measured or calculated: air and soil temperature, sunshine, wind speed, vapor pressure, saturation deficit, precipitation, humidity, incoming and reflected solar energy, energy emitted by active surfaces and primary production. Taking into account the relationships between climatological characteristics, the growth stages of vegetation, and relations between heat balance components, the fluxes of energy used for evapotranspiration, air, and soil heating were estimated in various ecosystems composing the agricultural landscape. The energy contained in biomass production of various crops was estimated also. Aggregate estimates of energy flow connected with evapotranspiration, and soil and air heating were calculated for eight model landscapes which differed by the plant cover structure. A higher variability of energy fluxes was observed for individual ecosystems than for agricultural landscapes. It was shown that the structure of the plant cover has an important bearing on energy flow and water cycling both by direct and indirect influences. Shelterbelts are especially important in their influence on energy flow and water cycling.

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Changes of body weight during pregnancy and lactation as well as food ingested, assimilated and metabolizable energy were recorded. Pregnancy lasted on an average 19.6 days, mean number of born animals was 9.78, and the mean body weight of newborn mouse was 1.14 g. The total amount of metabolizable energy taken by the female in connection with pregnancy was 82.32 Kcal. Caloric value of litter was 10.704 Kcal, placenta 4.004 Kcal. The body weight increase of female itself was observed during pregnancy and lactation. Caloric value of one gram of wet weight of the placenta was 513.7 cal and dry weight 6,306 cal/g. The total amount of metabolizable energy additionally taken by female during 26 days of lactation amounted to 354.224 Kcal.